BOULDER COUNTY — Poofs of smoke and coal ash wafted from a 30-foot pit on Marshall Mesa as an excavator the size of a small house scooped dirt and coal out of the ground.
The excavator broke chunks of coal from an underground seam and dumped them onto a pile of dirt and rock, where a bulldozer mixed it all together. A tanker truck occasionally blasted the hot coal with water to bring down the heat.
Ultimately, all of this debris will be returned to the ground and buried.
And, hopefully, a 100-year-old underground fire will breathe its last because the coal will be busted apart and smothered with dirt, reducing the potential for combustion.
“It’s really difficult for a fire to sustain itself because the grain-to-grain contact isn’t there,” said Jeff Graves, program director for inactive mines at the Colorado Division of Reclamation, Mining and Safety.
The mining division began extinguishing the underground coal-seam fire on Marshall Mesa in November, and the project is expected to be finished in January. Exhausting the fire became a priority after the smoldering coal seam was investigated as a possible cause of the devastating 2021 Marshall fire. It was eventually ruled out.
The Marshall coal fire is one of 38 listed in the mining division’s 2018 report on underground mine fires across Colorado. It was considered a low risk because it did not have high fire activity. The report, however, noted the fire was burning beneath a popular recreation area.
“The Marshall fire played a role because of the community input and concern,” Graves said.
The mining division this year finished a similar project to douse an underground fire at the Lewis Coal Mine fire off Cherryvale Road in unincorporated Boulder County.
That fire burned for years underneath the Davidson Ditch and monitors had noticed surface fracturing and cracking, increased venting and gas emissions, and stressed vegetation in recent years, although the threat of a wildfire was low, according to a project description on the Boulder County website.
The mining division is attacking underground fires over the next 12 years after receiving nearly $10 million annually from the federal Infrastructure Investment and Jobs Act. The Marshall Mesa coal fire will cost an estimated $870,000 to extinguish.
The coal-seam fire has burned for at least a century on Marshall Mesa, land that is now part of Boulder Open Space and Mountain Parks. The open space, which is wildly popular with hikers and mountain bikers, is accessible at a trailhead near the intersection of Colorado 93 and Colorado 170.
Last year, the park received 83,431 visits, said Phil Yates, a Boulder open space spokesman.
The Marshall fire ignited on Dec. 30, 2021, during an intense windstorm that relit smoldering embers from a week-old fire on the property of an international religious cult and also caused an Xcel Energy power line to snap and send sparks into dry grasses on Marshall Mesa, according to the investigation into the cause.
The fire killed two people and destroyed more than $2 billion in property, making it the most destructive wildfire in Colorado history.
The Marshall Mesa Trailhead was closed from Dec. 20, 2021, to Feb. 18, 2022, as the wildfire’s cause was investigated.
Boulder County District Attorney Michael Dougherty said the underground fire almost certainly did not start the deadly blaze. Still, people in the community wanted something done about it.
No one knows exactly what caused the fire to start burning underground, said Jeremy Reineke, the project manager for the Division of Reclamation, Mining and Safety.
Experts know the fire dates back at least to the 1920s because of a historical photograph of a boy standing on the mesa with smoke billowing from the ground and fractures in the earth where he stands.
Coal mines operated in the Marshall Mesa area from 1863 until 1939. It’s not known whether miners did something to cause the fire and just left it after failing to extinguish it, or if the coal spontaneously combusted after being hit with right mix of heat and oxygen.
The underground fire smoldered for years as employees at the mining division monitored it.
“It hasn’t necessarily been an extremely intense fire,” Graves said.
Over the years, monitors had noticed patches of snowmelt caused by the underground heat. But no one in recent memory has recorded venting or fracturing from the underground fire, he said.
“Coal is so insidious,” Reineke said. “It can sit there and smolder at low temperatures for a very long time with very little oxygen.”
On a recent weekday, the excavator hit an occasional spot that was smoldering, sending smoke upward. But no flames ever grew from the heat.
“Everybody has visions that you’re digging up an open charcoal grill with big burning flames,” Reineke said.
That doesn’t happen.
But the on-site workers are well aware of lingering fears from the Marshall fire and worries that smoldering coal and high winds could trigger another wildfire. Precautions are in place, Reineke said.
Employees at the site are constantly monitoring the pit with infrared cameras and thermometers to record temperatures. When the excavator hits a hot spot, a tanker truck sprays it with water. Coal needs to reach 700 degrees before it combusts, but so far the hottest spots have measured at 450 degrees.
There’s a weather station in an office trailer at Marshall Mesa, so workers can get real-time meteorological data. If winds hit 15 mph or higher, work stops to prevent sparks from flying through dry grass in the open space. That work stoppage in strong winds also prevents the spread of coal ash, which is made of fine particles that can affect people’s breathing.
The mining division also keeps 30,000 gallons of water in two on-site tanks should a fire break out. So far, that hasn’t happened, Reineke said.
The reclamation project should wrap up in early January, at which point state mining officials will turn the site back over to Boulder Open Space and Mountain Parks.
The open space department will spend an estimated eight months improving the trailhead’s parking area and vehicle and pedestrian access. There also will be additions to improve accessibility for those who have limited mobility, and new restrooms will be built along with a new shade shelter. And more picnic tables, bike racks and benches will be added, Yates said.
When the project is finished, the excavation team will have moved 182,000 cubic yards of material, which is about 72,000 pickup-truck loads of coal, rock and dirt.
And the team is saving some big chunks of coal and fossilized rocks to display at the park so visitors can experience the land’s history.
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